FIGS. 9 and 10 are cross-sectional views showing the structure of two different prior art monolithic SAW convolvers, in which reference numeral 1 is a high impurity concentration semiconductor substrate; 2 is an insulating layer; 3 is a piezoelectric film; 4 is a gate electrode; 5 is interdigital electrodes of an input transducer; 6 is a rear electrode; 7 is an input terminal; 8 is an output terminal; 9 is a high impurity concentration semiconductor substrate; and 10 is a low impurity concentration semiconductor epitaxial layer.
That is, the device indicated in FIG. 9 is characterized by a piezoelectric film/insulator/semiconductor structure and the device indicated in FIG. 10 by a piezoelectric film/insulator/low impurity concentration semiconductor epitaxial layer/high impurity concentration semiconductor substrate structure. Further, in the structure indicated in FIG. 10, the semiconductor epitaxial layer 10 and the high impurity concentration semiconductor substrate are made of a same material. Therefore the epitaxial layer has a same lattice constant as the semiconductor substrate and thus they form a so-called homo-junction.
Comparing FIG. 9 with FIG. 10, it is known that the structure indicated in FIG. 10 has a higher convolution efficiency F.sub.T and in the present state the structure indicated in FIG. 10 is used in practice. Various characteristics of convolvers having the structure indicated in FIG. 9 are described in detail in following literatures [1] and [2];
Literature [1]
B. T. Khuri-Yakub and G. S. Kino, "A Detailed theory of the monolithic zinc oxide on silicon convolver", IEEE Trans. Sonics Ultrason., vol. SU-24, No. 1, January 1977, pp. 34-43.
Literature [2]
J. K. Elliott, et al. "A wideband SAW convolver utilizing Sezawa waves in the metal-ZnO-SiO.sub.2 -Si configuration", Appl. Phys. Lett. 32, May 1978, pp. 515-516.
On the other hand, various characteristics of convolvers having the structure indicated in FIG. 10 are described in detail in following literatures [3] and [4];
Literature [3]
S. Minagawa, et al. "Efficient ZnO-SiO.sub.2 -Si Sezawa wave convolver", IEEE Trans. Sonics Ultrason., vol. SU-32, No. 5, September 1985, pp. 670-674.
Literature [4]
U.S. Pat. No. 4,757,226
Further another structure is known, in which not the low impurity concentration epitaxial layer/high impurity concentration semiconductor substrate structure, as indicated in FIG. 10, but inversely a high impurity concentration epitaxialy layer/low impurity concentration semiconductor substrate structure, where the epitaxial layer and the substrate are made of a same material, is used instead thereof. Concerning examples of this structure, refer to following literature [5];
Literature [5]
Kuroda, et al. "Analysis of propagation characteristics of SAW in ZnO/GaAs structure (in Japanese)" Acoustic Wave Device, 131st Committee, Science Promoting Association of Japan, Report of Research Subcommittee, Jan. 26, 1983.
However the structure described in Literature [5] has a drawback that the convolution efficiency F.sub.T is as low as that of the structure indicated in FIG. 9 and that it is not practical for a convolver.
That is, in the present state, as the prior art structure, only that indicated in FIG. 10 is used in practice owing to the high convolution efficiency F.sub.T thereof. In particular, it is known that a high convolution efficiency F.sub.T is obtained, in the case where ZnO is used for the piezoelectric film and Si for the semiconductor in the structure indicated in FIG. 10 and in fact, a ZnO/SiO.sub.2 /n-Si epitaxial layer/n.sup.+ -Si substrate structure is used in practice. This structure is described in detail in Literature [3] and Literature [4] stated previously.
However there is a drawback also in the prior art structure indicated in FIG. 10. It consists in the fact that, in order to obtain a sufficiently high convolution efficiency F.sub.T of an element and good temperature characteristics thereof, it is necessary to restrict the thickness L of the epitaxial layer with respect to the maximum width of the depletion layer Wmax so as to satisfy approximately Wmax&lt;L.ltoreq.Wmax+2 .mu.m. This indicates that for Si, it is necessary to restrict the thickness L of the epitaxial layer so as to satisfy L.ltorsim.several .mu.m. (This point is explained in detail also in Literature [4].)
In practice, in the case where a low impurity concentration epitaxial layer is grown on a high impurity concentration Si substrate at a thickness smaller than several .mu.m, since impurities are diffused from the high impurity concentration substrate side to the epitaxial layer, it is not easy to secure the reproducibility for the impurity concentration distribution and the thickness L of the epitaxial layer. As the result, fluctuations in characteristics of elements are great, which can be a cause of decreasing the yield of the fabrication of elements. That is, in the prior art structure, even the structure having the highest convolution efficiency F.sub.T, indicated in FIG. 10, has a drawback that the yield can be decreased, if the convolution efficiency F.sub.T is increased and temperature characteristics are improved.